1. Field of the Invention
This invention is related in general to the field of scanning interferometry and, in particular, to a motorized device for effecting the vertical scanning motion of a sample or of an optical microscope objective about a focal point.
2. Description of the Related Art
Vertical scanning interferometry (VSI) is a technique where white light is used as a source in an interferometer and the degree of fringe modulation, or coherence, of the interference fringes is measured for various distances between a test surface and the optics of the interferometer (each corresponding to a different optical path difference, OPD) to determine surface height. The method typically involves vertical scanning of the reference arm of the interferometer with respect to a stationary sample and calculation of the relative modulation of the intensity signal as a function of vertical position. VSI techniques have been used successfully in overcoming the limitations of surface height measurements encountered in conventional phase shifting interferometry.
As illustrated in simple schematic form in FIG. 1 and described in further detail in U.S. Pat. No. 5,204,734, herein incorporated by reference, typical vertical scanning interferometric equipment 10 comprises a white-light source 12 directing a beam L of white light through a conventional illuminator 14 toward a beam splitter 16, which reflects the light downward in the direction of a test surface S. The light reflected by the beam splitter 16 first passes through a microscope objective 22 focused on the test surface S, which incorporates an interferometer (such as Mirau) comprising a beam splitter and a reference mirror (housed in a reference arm not shown in the drawings), so that two light beams are generated for producing interference fringes as a result of the optical path difference between the reference mirror and the test surface S. Thus, as is well understood by those skilled in the art, the beams reflected from the reference mirror and the test surface S pass back up trough the microscope objective 22 and upward through the beam splitter 16 to a solid-state imaging array 24 positioned in a camera 26 in coaxial alignment with the objective 22. The imaging array 24 consists of individual charge-coupled-device (CCD) cells or other sensing apparatus adapted to record a two-dimensional array of signals corresponding to interference effects produced by the interferometer as a result of light reflected at individual x-y coordinates or pixels in the surface S and received at corresponding individual cells in the array. Appropriate electronic hardware (not shown) is provided to process the signals generated by each cell and transmit them to a computer for further processing. Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the array 24.
In vertical scanning interferometry, a profile of the surface S is produced by repeating the measurement at different, constant-interval distances between the objective 22 and the test surface S (that is, at different elevations of the scanning mechanism), so as to provide information concerning the variation of light intensity at each pixel as the corresponding optical path difference is varied systematically with respect to an initial reference point. Thus, the position of the scannning mechanism corresponding to maximum interference at each pixel is determined and used, based on the distance from the reference point, to calculate the height of the surface at that pixel. Therefore, either the objective 22 or the test surface S is moved vertically to produce these repeated measurements (vertical scanning). It is noted that the present description is based on the configuration of a Mirau interferometer but, as one skilled in the art would readily understand, it is equally applicable to any of the other instruments used in vertical scanning interferometry, such as Michelson, Linnik or Fizeau.
Interferometric scanning utilizes piezoelectric transducers (also known as PZT ceramics) to translate the sample or the reference arm in the interferometer (which is normally incorporated in the optical microscope objective), with respect to the fixed sensor. For white-light scanning interferometry, the scan range provided by PZT elements can be about 100 .mu.m, while for conventional phase-shifting interferometry scanning is typically limited to a few wavelengths of light, or less than 1 .mu.m. PZT translators are used because they provide very smooth motion with quick response over the small distances required for phase-shifting measurements and because they can be easily controlled by either closed-loop control techniques (such a by linear-variable-differential-transformer position sensing--LVDT) or by open-loop configurations.
One of the major disadvantages of PZT translators, though, is their limited range of operation, typically within 100 .mu.m, which greatly limits the application of vertical scanning techniques for profiling rough surfaces. Another drawback is the hysteresis effects inherent in the cyclical electromechanical operation of the PZT material, which requires closed-loop feedback or sophisticated control algorithms to ensure consistency of measurements. A third problem is the nonlinear response of piezoelectric elements to applied voltages, which also requires corrective control schemes to obtain reliable measurements. Finally, other disadvantages of piezoelectric transducers are high cost, high operating voltages (in the order of 100 or more volts), and the very delicate handling that they require.
As vertical scanning interferometry becomes a preferred method for measuring surface heights, a need has developed for instruments capable of scanning greater distances than PZT translators are able to cover. In addition, as greater scanning distances are spanned, the linearity of the scanning mechanism becomes more important in order to minimize the need for corrective measures. Therefore, this invention is directed at providing a broader-range, preferably linear scanning mechanism to increase the range of operation of white-light vertical scanning interferometry.